Electron microscopic demonstration of caudal afferents to the rat interpeduncular nucleus

Abstract

Analysis of the synaptology of the rat interpeduncular nucleus is of interest, amongst other reasons, because of morphological and biochemical evidence for synaptic sprouting following lesions of the habenular nucleus. Thus far only the synapses arising from the habenulointerpeduncular afferent axons have been identified by electron microscopy. The afferents from the midbrain tegmentum, which form a tegmentopeduncular tract, have been studied by the electron microscopic degeneration method in adult rats. Electrolytic lesions were placed stereotaxically so as to destroy the tegmentopeduncular tract, as well as various portions of the dorsal or ventral tegmental nuclei. The degeneration of endings in the interpeduncular nucleus was the same in each case. This consisted of (1) occasional neurofilamentous type degeneration of axons forming axodendritic synapses at 3 days' survival; (2) dense type degeneration of axons in contact with either small (0.4–0.8μm) or large (1.4–2.0μm) dendrites, with synaptic contacts which were symmetrical to moderately asymmetrical; (3) occasional axosomatic synapses also showing dense type degeneration. The projection was apparently greatest in the caudal portion of the interpeduncular nucleus. Equally important were observations of structures which were unaffected by the lesions. These included the S and crest synapses, some somatic synapses, and apparently normal numbers of synapses containing flattened vesicles. The present observations demonstrate the afferents from the tegmentum caudal to the interpeduncular nucleus at the fine structural level. A small number of those synapses previously classified as S or somatic synapses, and possibly as F synapses are of this origin. These afferents may be those which undergo sprouting caused by habenular lesions, with resulting formation of heterologous crest synapses, increase in number of somatic synapses, or both.